1. Field of the Present Invention
The present invention relates to the field of semiconductor circuits. More particularly, the present invention relates to high speed and low noise semiconductor output circuitry.
2. Description of the Prior Art
The circuit design of output circuitry is of particular importance for the desired functioning of semiconductor chips and systems. The semiconductor output circuitry should be designed such that it ca drive high capacitance loads at a high speed. Furthermore, the semiconductor output circuitry should not generate excessive "inductive noise" on the ground and the supply voltage lines, even when the output circuitry is driving a highly capacitive load. Moreover, the semiconductor output circuitry should maintain its high speed even at high temperatures, and further maintain its low noise characteristics even at low temperatures. Therefore, the design of semiconductor output circuitry is of critical importance to the proper functioning of a semiconductor chip or system.
FIG. 1 illustrates a recent version of a prior art semiconductor output circuit indicated generally by numeral 10. Referring to FIG. 1, output driver transistor 20 typically drives the output load capacitance 22. Gate 18 of output driver transistor 20 is driven by transistors 14 and 16. The voltage at gate 27 of transistor 16 is determined by internal calculations of the semiconductor chip. Similarly, node 19 is connected to an output driver transistor 30, which in turn drives the output node 21. Transistors 15 and 17 are responsible for driving output driver transistor 30. The inverse polarity of the data that is placed at gate 27 of transistor 16 is placed at gate 37 of transistor 17. A reference voltage Vref drives both transistors 14 and 15.
Now attention is turned to the operation of the typical prior art output circuit shown in FIG. 1. P-channel transistor 14 is responsible for both "pulling up" gate 18 of transistor 20 to the supply voltage and also for counterbalancing the effect of temperature variations on the speed and the noise level at the output node 21. To perform the "pulling up" function, the voltage at the gate 28 of transistor 14 is always kept low enough so that transistor 14 always remains in the conductive state. Therefore, transistor 14 always provides a conductive path from node 18 to the supply voltage. As such, gate 18 of the driver transistor 20 is pulled up to the supply voltage whenever the pull-down N-channel transistor 16 is in the non-conductive state.
As stated above, transistor 14 also performs the function of compensating for the effect Of temperature variations on the speed of, and the noise generated by, the prior art output circuit 10. This function is performed by varying the voltage at gate 28 of transistor 14 as a function of the variations in the ambient temperature. The variations in the voltage of gate 28 of transistor 14 in turn causes concomitant changes in the resistivity of transistor 14. More specifically, the voltage of gate 28 is typically increased in response to a decrease in the ambient temperature. This increase in the voltage of gate 28 causes a concomitant increase in the resistivity of P-channel transistor 14, thus reducing the current flow from transistor 14 to transistor 16 during transitions of gate 18 of transistor 20. As such, the otherwise usual effect of low temperatures, which is to increase the current flow in semiconductor circuitry, is counterbalanced by actually reducing the current flow in the output circuitry at low temperatures in the manner discussed above. Thus, the generation of excessive noise by the output circuitry 10 is hindered, in part, by controlling the resistivity of the P-channel transistor 14.
Likewise, the otherwise usual effect of a high ambient temperature, namely a decrease the current flow in semiconductor circuitry, is offset by increasing the current flow between transistors 16 and 14 at high ambient temperatures in a similar manner that was discussed above.
As discussed above briefly, transistor 16 of the typical prior art output circuit 10 performs a pull-down function. Thus, when gate 27 of transistor 16 is at a voltage higher than the threshold voltage of transistor 16, it will be placed in the conductive state. In this state, transistor 16 "pulls down" gate 18 of transistor 20. Of course, transistors 16 and 14 are sized such that when transistor 16 is in the Conductive state, the path that transistor 16 provides to the ground is less resistive than the path provided by transistor 14 to the supply voltage, thus allowing transistor 16 to pull down gate 18 of transistor 20 to the ground voltage.
Transistor 30, the counterpart to transistor 20, drives the output node 21 to the supply voltage (less the threshold voltage of the N-channel transistor 20). Transistor 30 is driven by transistors 15 and 17 in generally the same manner that transistor 20 was driven by transistors 14 and 16. Therefore, transistor 30 and its pre-driver 40 are essentially a duplicate of transistor 20 and its pre-driver 42. Thus, the detailed operation of transistor 30 and pre-driver 40 is not separately discussed.
The typical prior art output circuitry described above has some disadvantages that are overcome by the circuit of the present invention. For example, the speed, power consumption, and noise performance of the prior art circuit 10 are significantly improved by the circuit of the present invention as will be discussed in the "Detailed Description of the Present invention" section below.